GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2004 🢒 XLR 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 Horns Inoperative

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Verifies that the body control module (BCM) is providing ground to the horn relay.

    2.

  2. Tests for voltage at the battery positive voltage terminal of the horn relay coil.

    3.

  3. Tests for voltage at the battery positive voltage terminal of the horn relay switch. The Horn Battery Power fuse supplies power to the battery positive voltage terminal of the horn relay switch.

    4.

  4. The horns need maximum current flow to operate properly. A high resistance, greater than 0.5 ohms, in the horn control circuit or the horn ground circuit could cause operating problems with the horns. Inspect the circuits for faults that would restrict current flow.

    5.

  5. The horn relay control circuit includes the horn slip ring and the horn switch. The horn slip ring and horn switch must be tested for a high resistance, an open or a short to voltage when testing the relay control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Step

Action

Yes

No

1

Did you perform the Horns Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

Depress the horn switch.

Do the horns operate?

Go to Step 3

Go to Step 5

3
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With a scan tool, command the body control module (BCM) Horn Output ON and OFF.

Do the horns operate?

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 4

4

Test the BCM portion of the horn relay control circuit for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 11

5

  1. Turn OFF the ignition.
  2. Disconnect the horn relay.
  3. Connect a test lamp between the battery positive voltage circuit of the horn relay coil and a good ground.

Does the test lamp illuminate?

Go to Step 6

Go to Step 14

6
  1. Connect a test lamp between the battery positive voltage circuit of the horn relay coil and the horn relay control circuit.
  2. Depress and hold the horn switch.

Does the test lamp illuminate?

Go to Step 7

Go to Step 15

7

Connect a test lamp between the battery positive voltage circuit of the horn relay switch and a good ground.

Does the test lamp illuminate?

Go to Step 8

Go to Step 16

8

Momentarily connect a 15-amp fused jumper wire between the battery positive voltage circuit of the horn relay switch and the horn control circuit.

Do the horns operate?

Go to Step 12

Go to Step 9

9
  1. Reconnect the horn relay.
  2. Disconnect either horn connector.
  3. Connect a test lamp between the horn control circuit and a good ground.
  4. Depress and hold the horn switch.

Does the test lamp illuminate?

Go to Step 10

Go to Step 17

10
  1. Connect a test lamp between the horn control circuit and the horn ground circuit.
  2. Depress and hold the horn switch.

Does the test lamp illuminate?

Go to Step 13

Go to Step 18

11

Inspect for poor connections at the harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 19

12

Inspect for poor connections at the horn relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 20

13

Repair the high resistance in the horn control circuit or the horn ground circuit. A resistance as low as 0.5 ohms could cause operating problems with the horns. Also, though unlikely, it is possible that both horns are defective. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 21

--

14

Repair the open or high resistance in the battery positive voltage circuit of the horn relay coil. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 21

--

15

Repair the open or short to voltage in the horn relay control circuit. The horn relay control circuit includes the horn slip ring and the horn switch. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 21

--

16

Repair the open or high resistance in the battery positive voltage circuit of the horn relay switch. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 21

--

17

Repair the open or high resistance in the horn control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 21

--

18

Repair the open or high resistance in the horn ground circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 21

--

19

Important: Perform the BCM programming procedure when replacing the BCM. Refer to Body Control Module (BCM) Programming/RPO Configuration in Computer/Integrating Systems.

Replace the BCM. Refer to Body Control Module Replacement in Computer/Integrating Systems.

Did you complete the repair?

Go to Step 21

--

20

Replace the horn relay.

Did you complete the replacement?

Go to Step 21

--

21

Operate the horns.

Do the horns operate properly?

System OK

Go to Step 1